cmd/doc: avoid calling token.IsExported on non-tokens
token.IsExported expects to be passed a token, and does not check for
non-token arguments such as "C:\workdir\go\src\text".
While we're at it, clean up a few other parts of the code that
are assuming a package path where a directory may be passed instead.
There are probably others lurking around here, but I believe this
change is sufficient to get past the test failures on the
windows-amd64-longtest builder.
Fixes #35236
Change-Id: Ic79fa035531ca0777f64b1446c2f9237397b1bdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/204442
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Daniel Martà <mvdan@mvdan.cc>